How sintering works in powder metallurgy
Sintering is a heat treatment commonly used in powder metallurgy to enhance the strength and integrity of an object.
What is powder metallurgy?
Powder metallurgy is an umbrella term that encompasses a variety of manufacturing processes. What these processes have in common is that they use powdered feedstock in parts fabrication.
Sintering is a heat treatment commonly used to increase the strength and structural integrity of a given material. Powder metallurgy processes use sintering to convert metal powders and other unique materials into end-use parts. Sintering typically takes place in a single, elongated furnace with different temperature zones.
To facilitate the process, alloying elements or pressing lubricants may be added to the metal powder. In traditional solid state sintering, pressing lubricants are first eliminated via evaporation and/or combustion of the vapors. Oxides on particle surfaces must also be sufficiently reduced to allow the process to work.
The iron powders used in traditional powder metallurgy are often manufactured using high-pressure water atomization, a relatively inexpensive method used with metals that do not react with water. This method produces irregularly shaped particles.
Metal powders such as titanium, cobalt chromium and stainless steel are often produced using plasma atomization, which produces high quality, spherical particles for sintering in 3D printing processes. The consistency of particle sizes and shapes yields predictable results. For example, AP&C’s proprietary Advanced Plasma Atomization technology produces uniform powder size and batch-to-batch consistency.
The Sintering Process: Initial Stage
The sintering process takes place in furnaces that cool materials at a rate consistent with the creation of martensitic, crystalline structures.
At first, discrete particles remain because complete compaction or melting does not occur. In powder metallurgy, the selected sintering temperature is always lower than the melting point of the primary metal in the material. However, the selected temperature is high enough to promote neck formation at the point of contact between adjacent particles, although channels remain between the necks.
The consolidation process is accomplished in a variety of ways, including the use of a toolset to press particles closer together. Other consolidation processes include sintering in 3D printing, which involves the partial melting of powders via a laser or thermal print head.
In traditional sintering associated with powder metallurgy, particles are initially conjoined by cold welds. This gives the powder compact enough green strength to hold together during further processing.
The Sintering Process: Intermediate Stage
During the intermediate stage of the sintering process, the packing density increases as adjacent necks increasingly merge and particles diffuse. The resulting channel closures increase densification. Two common methods for accomplishing this are transient liquid phase sintering and permanent liquid phase sintering.
Transient liquid phase sintering
Transient liquid phase sintering may involve the addition of copper powder to iron powder to counter the natural pole shrinkage that would otherwise occur during the sintering process. At regular sintering temperatures, the copper fully melts, and the molten copper infuses and swells the iron particles. A carefully calculated amount of copper causes swelling that accurately counters the shrinkage that would otherwise occur. The molten copper further strengthens the object as it cools and hardens.
Permanent liquid phase sintering
The permanent sintering concept promotes capillarity that attracts the liquid into open pores, leading to grain movement that improves packing. Permanent liquid phase sintering is often used with materials such as cemented carbides.
The Sintering Process: Final Stage
In the final stage of the sintering process, pore shrinkage, decreases volume. Some isolated pores may remain, but the loosely packed material present early in the sintering process has largely been transformed into a solid state material.
In this final stage of permanent liquid phase sintering, liquid migrates from the more densely packed areas into the remaining pores. The successful rearrangement of particles into an ideally packed mass typically requires the melting of a binder additive.
Once sintering during the powder metallurgy process is complete, there are a variety of secondary operations that may be employed to further optimize performance, such as impregnation with resins or oils and infiltration with molten metals. Secondary operations also include various forms of surface engineering, like machining, shot peening, steam treatment, vapor deposition and high-velocity thermal spraying.